Hamsters self-administer androgens. Previously, we determined that testosterone (T) activates select steroid-and opiate-sensitive brain regions. Is T-stimulated neuronal activation androgenic? 35 castrated males with physiologic T replacement (n=7/group) were pre-treated with the androgen antagonist flutamide (15 mg/kg sc) or ethanol (0.25 ml), and infused into the lateral ventricle (ICV) for 4h with 40 μg T (TF and TE, respectively) or 40 μl vehicle (VF and VE). To determine if androgens and opiates activate overlapping brain areas, 7 additional males received 20 μg morphine sulfate ICV following ethanol injection (ME). Immediately after ICV infusion, animals were perfused. 60 μm coronal brain slices were stained for Fos. Fos-positive neurons were counted in a 0.3 mm 2 area from 5 regions previously shown to express T-induced Fos: the posteromedial bed nucleus of the stria terminalis (BSTPM), posteromedial amygdala (MeP), lateral habenula (LHb), ventral tegmental area, and lateral pontine nucleus. T induced Fos in all areas reported previously (TE vs. VE, p<0.05), except LHb (p>0.05). Morphine induced Fos in all 5 brain regions (ME vs. VE, p<0.05), indicating that androgens and opiates activate overlapping brain regions. Flutamide alone did not induce Fos (VF vs. VE, p>0.05). Moreover, flutamide treatment blocked T-induced Fos expression only in the steroid-sensitive BSTPM, suggesting that androgens mediate neuronal activation in this area (mean ±SEM: TF: 68.4±13.2 vs. TE: 137.9±17.6, p<0.05). The absence of flutamide effects on T-induced Fos in the steroid-sensitive MeP (TE: 210.6±50.0 vs. TF: 215.3±28.2, p>0.05) suggests that distinct mechanisms activate Fos in individual androgen-responsive nuclei.